The suspension is a component on a conventional cone driver. It is also known as the spider or damper. Cone drivers are widely used, particularly for the low (20-500 Hz) and midrange (500-3000 Hz) parts of the audio spectrum.
The suspension is typically used in conjunction with the surround—a flexible air seal between the cone and chassis. Together these centre the voice coil in the magnet gap, maintain axial travel, and provide a restoring force on the moving parts. Usually the suspension provides the greater portion of this restoring force. Over small excursions this force is fairly linear and influences the resonant frequency of the drive unit. Over larger excursions its behaviour is non-linear and may be characterised by a stiffness-displacement ‘K(x)’ curve. To avoid excessive displacement where moving parts collide with the chassis or magnet assembly, stiffness must increase for both positive and negative displacement. If the stiffness-displacement curve is not symmetrical about zero displacement the restoring force provided will not be equal for forwards and backwards motion and the voice coil will oscillate about a position that is offset from the centre of the magnet gap. At higher excursions much of the coil may be away from the cooling effect of the iron poles and may fail due to over-heating. A smooth symmetrical increase of stiffness with displacement reduces excessive excursions to minimise the distortion caused by motor nonlinearities.
The suspension is commonly an annular band attached to the voice coil former on its inner edge and the driver chassis on its outer edge. Its structure is often a series of concentric corrugations or ‘rolls’ of material. The number, size, and shape of the rolls greatly affect the stiffness-displacement curve.
The suspension is typically manufactured from a woven fabric impregnated with resin and moulded into shape. The material needs to be flexible and have some damping properties to minimise resonance in the working bandwidth. It will ideally be porous to avoid radiation of the resonance.
If the inner and outer edges attach at similar heights in the driver assembly the overall form of the suspension will be planar. In that case, the concentric rolls may be designed to provide a symmetrical stiffness-displacement curve.
In certain driver designs the inner and outer edges are not attached at similar heights, however. For instance, for manufacturing reasons the mounting surface on the chassis may not be located at the same height as the mounting position on the voice coil former. Most commonly the corrugated part of the suspension would be aligned to the most appropriate position on the voice coil former and the inner edge mounted there. The outer edge would protrude backwards to meet its mounting surface on the chassis.
An alternative example is a driver in which the suspension attaches to the diaphragm rather than the voice coil former to minimise overall build height. The corrugated part of the suspension is then aligned with the chassis mounting surface and the outer edge mounts there. The inner edge protrudes forward with a generally frusto-conical geometry to mount on the rear of the diaphragm. With this arrangement the rolls will not hit the diaphragm as it moves.
It would also be possible for both edges to protrude or have a shift in height within the corrugated part of the suspension.
These non-planar arrangements are sometimes described as cupped suspensions, where the protrusion is the ‘cup’. Such an arrangement may be employed for any number of design reasons.
The problem with a cupped suspension is that the cup can bend more easily in compression than it can stretch in extension. As a result the stiffness-displacement curve is asymmetric, with the restoring force much lower as the diaphragm moves in the direction of the protruding cup. As mentioned above this is an undesirable characteristic.